GB 1 255 555 A discloses a movable grate, for a furnace or an incinerator, in the form of steps, wherein each step is rockable by a drive mechanism to impart a wave-like motion to the material on the grate. Synchronizing means between juxtaposed steps maintains a predetermined clearance between the adjoining edge portions of those steps. The grate is divided into an upper and lower section, there being a drive mechanism for each section and each section has a different rate of movement.
WO 89/04441 discloses a movable grate comprising a number of grate steps which are arranged adjacent each other, partly overlap one another and are pivotal about an axis extending in the longitudinal direction of said grate step, and which are pivotally mounted outside shield members which in lateral direction enclose a combustion chamber. End plates are rigidly secured to the ends of the grate steps and pivotal therewith; the end plates being aligned with and fitted in openings in the shield members. Portions of the shield members having openings aligned with the end plates are displaceably mounted relative to adjoining shield members in the direction of the grate step axis, and the shield portions which radially outwardly sealingly engage adjoining shield members and radially inwardly sealingly engage the end plates, are in the direction of said axis held in a fixed position in relation to the grate step shaft.
WO 99/63270 discloses a grate device for a combustion furnace comprising a grate element and a turnable shaft assembly connected thereto. The grate element has a first system of ducts for circulating coolant through the grate element. The shaft assembly has a second system of ducts, which communicates with the first system of ducts and forms a coolant inlet and outlet. The grate element comprises a girder means which is non-rotatably connected with the shaft assembly and which contains a part of the first system of ducts, which part communicates with the second system of ducts. The grate element comprises a plate means which is mounted on the girder means and forms a grate area and through which the remaining part of the first system of ducts extends for cooling the grate area.
In these known devices, the grate bars on each grate shaft coincide with the grate bars on the neighbouring shaft without touching these, thereby forming a cohesive grate surface. The gap between two coinciding grate bars may for instance be approximately 1 to 3 millimetres. The grate function is such that the grate shafts alternately turn to their respective outer positions, and the grate surface thus forms a stair-shaped surface where the steps change direction. This produces a rolling movement to material present on the grate, which may have the effect of breaking it up and agitating it, while at the same time moving it forward in downward direction, thus achieving good exposure to radiant heat from a combustion chamber and good exposure to combustion air.
In addition to the above-mentioned grate devices, devices are known, wherein two grates of the above described type are arranged side by side and so that the grate device is composed by two grate lanes connected by means of a midsection. Thereby, the two grate lanes are arranged symmetrically so that the drive mechanisms are arranged along the outer free sides of the arrangement in order to provide for a slim midsection between the two grate lanes and in order to ensure easy access in connection with service and maintenance. In this way, a larger grate width and better flexibility may be obtained. The latter may be achieved due to the possibility of operating each grate lane independently, whereby the individual speeds of the grate lanes may be adapted to the amount of material present of the individual grate lanes. However, in these devices it is of importance that the midsection is relatively slim, because the midsection does not provide any movement to material present thereon, and no exposure to heat or combustion air is provided thereby. Furthermore, it is of importance that the drive mechanisms are not freely exposed under the grate lanes in order to reduce maintenance and in order to provide access to the drive mechanisms even during operation of the furnace, in the case that maintenance is necessary.
In order to achieve even larger grate widths and good flexibility, it would be desirable to combine even more than two grate lanes into one unit.